Certain naturally-occurring materials such as oil shale, coal, tar sands, and diatomaceous earth contain a carbonaceous fraction. During retorting, these carbon-containing solids release an oil useful in petroleum processing. Oil shale is a shale-like rock containing an organic component, usually referred to as kerogen, which upon heating releases volatile hydrocarbons that may be recovered as shale oil. A residual carbonaceous material typically remains with the inorganic component left following pyrolysis.
The pyrolysis or retorting process may be carried out in a retorting vessel of various designs. In one method the raw oil shale is crushed into particulate material containing a particle size distribution not exceeding about 2-1/2 mesh (Tyler Standard Sieve) which is fed into the upper portion of the retorting vessel. The raw oil shale is mixed with hot solid heat carrier particles which, along with the raw shale, moves downward as a continuous bed of material through the retorting vessel. An upward flow of a non-oxidizing stripping gas counter-current to the downward moving shale carries the volatile hydrocarbons away from the bed. See U.S. Pat. No. 4,199,432. In order to prevent gross vertical backmixing and slugging of solids passing downward with the bed, the retorting vessel is equipped with a plurality of dispersing elements in the form of perforated plates, bars, screens, packing, or other suitable internals. The dispersing elements also serve to limit the size of gas bubbles passing up the retorting vessel.
In operating the retorting vessel it is important to remove the vapor products without thermal cracking if high liquid yields are to be obtained. One way of minimizing thermal cracking is to operate the retorting vessel at a relatively low temperature. However, lower temperatures require longer retorting times, which means increased residence time in the retort. A longer residence time requires a larger retort or a decrease in the amount of raw shale that can be processed. The present invention is directed to a method for efficiently pyrolyizing the raw shale with a minimum of thermal cracking.